This invention is directed to (meth)acrylate ester-modified organopolysiloxanes composed of several fractions of equilibrated (meth)acrylate ester-modified organopolysiloxanes with differing average numbers of silicon atoms in the mixture of molecules. In particular, the invention is directed to (meth)acrylate ester-modified organopolysiloxane mixtures with improved adhesive properties.
The term (meth)acrylate ester-modified organopolysiloxanes is used to include acrylate ester-modified as well as methacrylate ester-modified organopolysiloxanes.
(Meth)acrylate ester-modified organopolysiloxanes are used to an appreciable extent as abhesive coating compositions. The modified organopolysiloxanes, if necessary in the form of a solution in a suitable solvent, are applied on a support, such as satinized paper and, in the event that solutions of the organopolysiloxanes are used, cured by electron beams after the solvent has evaporated off. The aim is to form a layer which adheres well to the support and cannot be removed from this support even by vigorous rubbing. The layer formed is intended to be abhesive towards adhesives, so that labels or adhesive tapes which are provided with adhesive can be pulled off with the least possible force without losing their own adhesive force in the process.
Suitable (meth)acrylate ester-modified organopolysiloxanes are known. From the many relevant publications and patents, German Patent 2,747,233 is singled out for mentioning. This patent discloses a method for the preparation of organopolysiloxanes modified with (meth)acrylate esters by the reaction of (meth)acrylate esters having --COH groups with organopolysiloxanes which have SiX groups (X=alkoxy, hydroxyl or chloro), optionally in the presence of catalysts. This method is characterized in that the organopolysiloxanes used have the formula ##STR1## wherein R.sup.1 is an alkyl group with 1 to 4 carbon atoms and/or a phenyl group;
X is a chloro or OR.sup.2 group; PA0 R.sup.2 is an alkyl group with 1 to 4 carbon atoms and/or hydrogen; PA0 a=1.0 to 2.0; PA0 b=0.02 to 1.6 PA0 a+b.ltoreq.2.66. PA0 R.sup.2 is a linear, branched or cyclic alkyl group with 5 to 18 carbon atoms, which optionally may be substituted by halogen groups, PA0 R.sup.3 is an aralkyl of the general formula ##STR3## wherein R.sup.4 groups are the same or different and represent hydrogen, alkyl groups with 1 to 12 carbon atoms and halogen, PA0 R.sup.5 groups are linear or branched alkylene groups with 1 to 8 carbon atoms, PA0 Q is pentaerythritol, trimethylolethane, trimethylolpropane or glycerin, all the hydroxyl groups except one of which are esterified with (meth)acrylic acid, as well as the dimers of these esters and/or the oxyalkylene derivatives with 1 to 10 oxyethylene and/or oxypropylene units, PA0 a.gtoreq.0.9, PA0 b, c=0 to 1.1, PA0 b+c.gtoreq.0.1, PA0 a+b+c=1.8 to 2.2, PA0 d=0.001 to 1.6 PA0 a+b+c+d.ltoreq.3. PA0 R.sup.2 is the same as R.sup.1 or denotes the R.sup.3 group, up to 70 to 100% of the R.sup.3 groups being epoxy-functional groups and up to 30 to 0% of the of the R.sup.3 groups being alkyl groups with 2 to 10 carbon atoms or hydrogen groups, with the proviso that at least 1.5 epoxy groups are contained in the average molecule PA0 a has a value of 1 to 1,000 PA0 b has a value of 0 to 10. PA0 60 to 95% by weight of a modified organopolysiloxane A comprising, on the average, 5 to 50 silicon atoms and PA0 5 to 40% by weight of a modified organopolysiloxane B comprising, on the average, at least 50 silicon atoms,
The above siloxane molecule contains 3 to 100 silicon atoms and pentaerythritol (meth)acrylate is used as the (meth)acrylate ester. Based on the COH- and SiX- groups, 0.05 moles to equimolar amounts of the pentaerythritol ester are used.
European Publication 0 254 799 discloses compounds of the general formula ##STR2## wherein R.sup.1 is a methyl group,
These compounds are suitable particularly as abhesive coating compositions or as casting compositions for electronic components.
Further (meth)acrylate ester-modified organopolysiloxanes, which have abhesive properties and are curable by radiation, are disclosed in European Publication 0,281,681. These materials are modified organopolysiloxanes, which are obtainable by the reaction of epoxy-functional polysiloxanes of the general formula ##STR4## wherein R.sup.1 groups are the same or different lower alkyl groups with 1 to 4 carbon atoms or phenyl groups,
The foregoing compounds are used with such amounts of an acid mixture, comprising 10 to 90 mole % of (meth)acrylic anhydride and 90 to 10 mole % of (meth)acrylic acid, the sum of the two adding up to 100 mole %, that there are 0.8 to 1.9 n acid equivalents for n epoxide equivalents, at elevated temperatures, if necessary in the presence of solvents and conventional esterification catalysts.
A modified organopolysiloxane which is to be used for the preparation of abhesive coatings must meet a series of requirements, some of which are difficult to fulfill. For example, a coating composition must adhere to the substrate, so that it can not be separated by mechanical means from the substrate. An insufficiently adhering coating composition can be removed from the substrate, for example, by rubbing. Moreover, if the adhesion is inadequate, the coating composition is detached partly or completely when the label, which is on the coating composition and is provided with adhesive, is pulled off. In such an eventuality, the adhesive force of the label is reduced or nullified. For this reason, the abhesive coating, when cured, must have adequate strength, so that the coating is not destroyed. Finally, a high abhesiveness towards the supports, such as labels, which are provided with adhesive and are on the coating, is of particular importance. This combination of properties, such as adhesion to the substrate, abhesiveness towards the support that is provided with adhesive and mechanical strength of the cured layer, are difficult to combine in only one (meth)acrylate ester-modified organopolysiloxane. Attempts have therefore already been made to achieve this desired combination of properties by the use of a mixture of (meth)acrylate ester-modified organopolysiloxanes.
The object of the German Patent 3,426,087 which corresponds to U.S. Pat. No. 4,678,846, the two patents hereinafter collectively being referred to as German Patent 3,426,087 are (meth)acrylate ester-modified organopolysiloxane mixtures, which are characterized by an equilibrated organopolysiloxane with, on the average, more than 25 to fewer than 200 silicon atoms, moreover of 2 to 30% by weight of organopolysiloxanes with, on the average, 2 to 25 silicon atoms and of 2 to 30% by weight of organopolysiloxanes with, on the average 200 to 2,000 silicon atoms.
In practice, it has turned out that the reproducible preparation of a mixture consisting of three different (meth)acrylate ester-modified organopolysiloxanes creates difficulties. Inherently, fluctuations always occur in the distribution of the individual components of such preparations and as a result, the property profile of these products is changed. It is difficult and complicated to make compensations for these changes in the mixtures.